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Question on Tube Swapping and Biasing

First time tube swapper. I’ve had my Luxman MQ-88use and CL-38use preamp for about 5 years. They sound fine, but it’s good to have a backup set of tubes. So I bought a matched set of 4 Gold Lion KT88 tubes and replaced the stock JJ tubes.

So far so good, everything sounds good, and hopefully even better after breaking in.

Should I be concerned about not setting the bias for these new tubes? And how would I go about doing that? The Luxman manual says nothing about this.

Some online sources recommend having an audio professional bias the tubes, but to my limited understanding the bias is affected by the actual current going into the amp, so having a servicer bias it at a shop might not be the optimal way to go.

Could I hurt the amp or the tubes my not properly biasing them? Or am I overthinking all of this?

jaylat
ghdprentice's avatar
ghdprentice
9,496 posts
 

It's really not complicated. But the reason why contemporary tube amps have auto biasing circuits so no manual intervention is required. 

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elliottbnewcombjr
7,825 posts
 

Getting it right makes sense of course,

I would like external adjusters, a bias meter built in, or auto-bias, however I need 16 ohm taps, and only the MK1 has them, and it has scary internal adjusters/no meters which is why I am happy to live only 35 minutes from VAS.

oddiofyl's avatar
oddiofyl
4,591 posts
 

I use the VHT bias meter on my Quicksilver Mid Monos.    One amp at a time.  That tool's name Tube Tester is a misnomer.    VHT should call it a bias tester .

My amps use an LED and pot to adjust externally but it's much better to read the bias over when an LED starts to glow.   My last amp had internal bias test point and this was much easier and safer .

 

 

westcoastaudiophile
819 posts
 

I use plug-in bias tester, similar w/ case to https://www.amazon.com/Nobsound-Current-Probes-Amplifier-Cathode/dp/B0777F8DBK

the problem of measuring resistor voltage drop to setup bias is relatively inaccurate resistance, 5%..10%, which degrades with time. 

in order not to leave any fat / or other stuff on glass, tubes need to handle in clean cloth gloves.

more aggressive tubes like gold lion degrade performance significantly after 50..500hrs, while JJs are softer initially, but still strong after 3000hrs, and that’s why Luxman choses them. there is also improved KT88 variant for valuable amps to consider: https://www.telefunken-elektroakustik.com/product/kt88-tk-vacuum-tube/

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